The use of filter dyes in photographic elements is well known. When incorporated in the radiation sensitive layer, filter dyes can improve sharpness by absorbing light scattered from one silver halide grain to another. Filter dyes can also be used in antihalation layers to retard the sensitivity of one light sensitive layer to another in a multilayer element by absorbing unwanted radiation due to reflection or refraction.
In these as well as the many other uses for filter dyes in photographic elements it is important that the dyes do not wander or diffuse into adjacent layers. This can cause problems such as speed loss or stain in the adjacent layers. Filter dyes must also be completely decolorized or removed from photographic elements, or both, usually during processing when their function is complete, or staining results. The latter problem is compounded when a polymeric mordant is used in the filter layer to prevent dye wandering.
Filter dye stability, particularly at high temperatures and humidity conditions is also important. It is often also highly desirable that the filter dye has a steep absorption peak; that is, that the dye is "sharp cutting".
One method used to incorporate soluble filter dyes into photographic film element layers is to add them as aqueous or alcohol solutions. Dyes introduced by this method often wander into other layers of the element, usually with deleterious effect. While the use of polymeric mordants can prevent dye wandering, such mordants aggravate the stain problem encountered when the dye remains in the element through processing.
Filter dyes have also been prepared as conventional dispersions in aqueous gelatin using standard colloid milling or homogenization methods or as loaded latexes. More recently, ball-milling, sand milling, media-milling, and related methods of producing fine particle size slurries and suspensions of filter dyes have become standard tools for producing slurries and dispersions that can readily be used in photographic melt formulations. Solid particulate filter dyes introduced as dispersions, when coated at sufficiently low pH, can eliminate problems associated with dye wandering. However, filter dye dispersions prepared by milling tend to have slow wash-out rates resulting in lengthy processing or unacceptable dye staining. Further, milled particulate filter dyes provide relatively low absorption coefficients, requiring that an excessive amount of dye be coated. Among other things, milled dyes can also provide too broad an absorption envelope for the contemplated use; the wavelength of maximum absorption may not be at optimal position in the visible region, or the long-wavelength absorption edge of the absorption band may not be optimally placed for the intended application.
In addition, the time and expense involved in preparing serviceable solid particulate filter dye dispersions by milling techniques are a deterrent to their use, especially in large volume applications.
It is also known that oxonol filter dyes can be prepared and purified in the fully protonated form by dissolution with a base followed by reprecipitation in excess concentrated acid as described in U.S. Pat. No. 4,855,221. Such macroscopic processes yield agglomerated particulates which are highly suitable for the bulk solid storage of the dyes but are not suitable for use as photographic filter dye dispersions. Extensive treatment by mechanical milling techniques using dispersing aids and stabilizers such as roller milling, sand milling, ball milling, and the like are required before such dyes can be used for photographic filter dye applications. Accordingly, agglomerates and slurries prepared by macroscopic precipitation processes are not suitable for use as photographic filter dyes and fall outside the scope of the definition of a colloid dispersion useful for such purposes.
It is therefore desirable to provide solid particulate filter dye microdispersions that do not require mechanical milling before use and that do not wander but that wash out easily during processing leaving little or no residual stain. It is also desirable that such filter dye dispersions provide high light absorption efficiencies with sharp-cutting absorbance peaks.